1. Field of the Invention
This invention generally relates to improved structures for organic light emitting diodes (OLEDs), and more particularly to so-called top-emitting OLEDs.
2. Related Technology
Organic light emitting diodes (OLEDs) fall into two basic categories, bottom emitting devices and top emitting devices. Bottom emitting devices are fabricated on a substrate, typically glass, over which is deposited an ITO (indium tin oxide) anode, followed by OLED material and then a substantially opaque cathode. When electrically stimulated such an OLED emits light from the OLED material through the semi-transparent ITO layer and the glass substrate.
Heretofore substantially all practical devices have been of this type although such a structure exhibits some significant disadvantages. Firstly absorption and reflection losses within the substrate reduce the efficiency of such a device. However perhaps more importantly where, as in active matrix devices, thin film driver circuitry is associated with an OLED this reduces the light-emitting area available for a pixel of a display, since this circuitry is normally light-sensitive. It is therefore desirable to be able to fabricate so-called top emitters in which the anode is opaque and the cathode substantially transparent, so that light is emitted through the cathode rather than through the anode. In the case of an active matrix display this allows substantially all the area of a pixel to be occupied by light emitting material, and also allows a greater area to be allocated to thin film driver transistors, consequently increasing device efficiency and OLED lifetime (since a smaller current density may be employed for the same total light output). However although device structures have been proposed for top emitting OLEDs these have practical difficulties, as discussed further below.
FIG. 1 shows a vertical cross section through an example of a top-emitting OLED device, in this example comprising part of an active matrix display and thus including associated drive circuitry. The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a glass substrate 102 supporting a plurality of polysilicon and/or metallisation and insulating layers 104 in which the drive circuitry is formed. The uppermost layer of this set of layers comprises an insulating and passivating oxide layer (SiO2) over which an anode layer 106 is deposited. This anode layer may be formed from ITO (indium tin oxide), for example where the drive circuitry in a layer 104 only occupies part of an area of a pixel and it is desired to provide a substantially transparent device emitting from both sides. However one advantage of a top-emitting device is that the anode need not be transparent and may comprise a conventional metal layer such as a platinum layer.
One or more layers of OLED material 108 are deposited over an anode 106, for example by spin coating and subsequent removal of organic material from unwanted regions (by, for example, laser ablation), or by selective deposition, for example using an inkjet-based deposition process (see, for example, EP0880303). Organic LEDs may be fabricated using a range of materials including polymers, dendrimers, and so-called small molecules, to emit over a range of wavelengths at varying drive voltages and efficiencies. Examples of polymer-based OLED materials are described in WO90/13148, WO95/06400 and WO99/48160; examples of dendrimer-based materials are described in WO02/066552; and examples of small molecule OLED materials are described in U.S. Pat. No. 4,539,507. In the case of a polymer-based OLED layers 108 comprise a hole transport layer 108a and a light emitting polymer (LEP) electroluminescent layer 108b. The electroluminescent layer may comprise, for example, PPV (poly(p-phenylenevinylene)) and the hole transport layer, which helps match the hole energy levels of the anode layer and of the electroluminescent layer, may comprise, for example, PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene).
A multilayer cathode 110 overlies the OLED material 108 and, in a top-emitting device, is at least partially transparent at wavelengths at which the device is designed to emit. For a polymer LED the cathode preferably has a work function of less than 3.5 eV and may comprise a first layer having a low work function, for example a metal such as calcium, magnesium or barium, and a second layer adjacent the LEP layer 108b providing efficient electron injection, for example of barium fluoride or another metal fluoride or oxide. The top layer of the cathode 110 (that is the layer furthest from LEP layer 108b) may comprise a thin film of a highly conductive metal such as gold or silver. Metallic layers having a thickness of less than 50 nm, more preferably less than 20 nm have been found to be sufficiently optically transparent although it is preferable that the sheet resistance is kept low, preferably less than 100 ohms/square, more preferably less than 30 ohms/square. The cathode layer may be used to form cathode lines which can be taken out to contacts at the side of the device. In some configurations the anode, OLED material, and cathode layers may be separated by banks (or wells) such as banks 112 formed, for example, from positive or negative photoresist material. Banks 112 have an angle of approximately 15° to the plane of the substrate (although in FIG. 1 they are shown as having steep sides for ease of representation).
Broadly speaking there are five main criteria which suitable cathode electrode structures should aim to meet: transparency, a low series resistance to allow charge injection into the organic electroluminescent material, sufficient lateral conductivity to facilitate matrix addressing, encapsulation of the underlying organic layer(s) to protect it (them) from physical and chemical damage, and a deposition process which does not significantly damage the underlying organic layer(s). Since no single material has yet been found which meets all of these criteria the top emitter structures which have been published to date are multilayer structures (see, for example, U.S. Pat. No. 5,739,545, U.S. Pat. No. 5,714,838, WO99/31741, WO98/07202, U.S. Pat. No. 6,316,786, JP08185984, U.S. Pat. No. 5,457,565 and U.S. Pat. No. 5,429,884). For example U.S. Pat. No. 5,739,545 discloses a structure comprising an anode and cathode sandwiching an electroluminescent layer, the cathode layer comprising a thin metal layer, for example of calcium or MgAl, followed by a protective layer of wide band gap semi-conductor, for example zinc selenide (ZnSe), zinc sulphide (ZnS) or ZnSxSe1-x, and optionally a further layer of non-reactive metal or other conductive material such as aluminium, ITO or AlZnO. Such a structure is advantageous because both calcium and zinc selenide can be deposited by essentially damage free vapour deposition rather than by the sputtering which ITO requires.
Top-emitting and bottom-emitting OLED structures suffer from different problems. In bottom-emitting structures, in which the anode is transparent and the cathode comprises an opaque layer of metal, problems can arise resulting from the transmission of ambient light through the transparent anode into the device, where it reflects off the cathode and back out of the device in competition with the electroluminescent emission, thus reducing the contrast of the display. To address this problem it has been proposed that ambient light reflection from the cathode is reduced by incorporating an anti-reflection structure into the cathode, as described in Applied Physics Letters, vol 82, (16), 2715, U.S. Pat. No. 5,049,780, and WO01/08240. Other methods for improving the contrast of a bottom-emitting OLED device include the use of a circular polariser (see, for example, U.S. Pat. No. 6,211,613, to the present Applicant), and the use of a light absorbing material in the cathode (see, for example, WO00/35028 to the present Applicant).
In a top-emitting device, however, where it is desirable to meet the aforementioned criteria for a cathode electrode structure, one problem which arises is that of extracting the maximum amount of electroluminescently emitted light from the device rather than that of preventing ambient light falling on the device from escaping. Thus to improve the efficiency of top-emitting devices it is desirable to improve the efficiency with which photons generated within the organic electroluminescent layer 108 can be conducted through the cathode structure and out of the device towards an observer.